The present invention relates to a glucose concentration measuring apparatus which measures a glucose concentration of a sample such as tissues of an organism using an optical device, and which can eliminate the syringe sampling of blood.
In recent years, there have been proposed so-called non-invasive type apparatuses for measuring a blood sugar level, which apparatuses are used to diagnose and care for diabetes patients. These apparatuses measure a glucose concentration in blood or an organism's tissues by use of an optical device without collecting blood or fracturing a part of the organism's tissues.
The basic concept of optical glucose concentration measurement is: Two kinds of radiation are projected to a sample to be measured. One kind is of radiation which is absorbed by glucose, and absorption of which changes according to glucose concentration. The other is of radiation which is hardly absorbed by glucose. A glucose concentration of a sample is obtained based on the energy of radiation transmitted through or diffused/reflected by the sample while excluding the influences by components other than glucose.
European Unexamined Patent Publication No. EP-A-160768 discloses: near infrared radiation having a wavelength .lambda.G of any of 1575 nm.+-.15 nm, 1765.+-.15 nm, 2100 nm.+-.15 nm and 2270.+-.15 nm is selected as radiation to be absorbed by glucose and near infrared radiation having a wavelength .lambda.R selected from a wavelength range between 1000 nm and 2700 nm is selected as radiation hardly to be absorbed by glucose.
According to the disclosure of EP-A-160768, radiation of a measurement spectral range including a wavelength .lambda.G and radiation of a reference spectral range including the wavelength .lambda.R are projected to a sample to be measured. Light energies transmitted through the sample are respectively received. An absorbed quantity of near infrared radiation of the measurement spectral range, namely a measurement value, and an absorbed quantity of near infrared radiation of the reference spectral range, namely a reference value, are compared so as to reduce the influence of errors caused by light absorption by substances other than glucose.
Further, Japanese Unexamined Patent Publication No. 3-173535 discloses a measuring method on the basis of the same concept as the above publication. Near infrared radiation of a wavelength range between 1600 nm and 1750 nm is used as radiation to be absorbed by glucose and near infrared radiation of a wavelength range between 1200 nm and 1300 nm is used as radiation not to be absorbed by glucose.
Moreover, Japanese Unexamined Patent Publication No. 3-146032 discloses non-invasive type method and apparatus for measuring a blood sugar level. According to this disclosure, radiation of a measurement wavelength range between, e.g., 1.3 .mu.m and 1.9 .mu.m is projected at a sample to be measured and a blood sugar level is calculated based on the obtained level of received radiation and a prestored verification curve.
Furthermore, U.S. Pat. No. 5,086,229 discloses, on the basis of the fact that a level of received measurement radiation differs depending upon temperature, more accurate measurement of glucose concentration by adding a factor, namely, a surface temperature of a sample to be measured into an operation expression used to calculate a glucose concentration.
There are also proposed measuring methods which, in order to measure the concentration of substances in blood more accurately, use a pulsation component and a variation resulting from an increased or reduced pressure (Japanese Unexamined Patent Publications Nos. 4-40940 and 4-332535).
The invention disclosed in U.S. Pat. No. 5,086,229 is capable of measuring a glucose concentration with the reduced influence of temperature because a temperature factor is taken into account. However, a special device for measuring surface temperature must be provided.
In addition, the surface temperature of an organism does not necessarily correspond with the temperature of blood or in the organism, or there may be no correlation therebetween.
Thus, it cannot be denied that there is an accuracy limit in measuring glucose concentration Further addition of a measurement factor caused by measuring the surface temperature of an organism complicates the processing for measurement data and a circuit therefor and adversely influences measurement accuracy.